Tried and tested businesses that require little to no capital investment?

What are some types of businesses that are known to generate money which require little to no investment to start?

There are probably a number of businesses that have been proven successful with little money to start but that doesn't mean they are guaranteed or that they don't take a heavy time investment.

• Freelancing is probably one of the better known business types that require little money to start but it usually take a big time investment to source sufficient clients and hone a skill.
• The $100 Startup by Chris Guillebeau highlights businesses that took very little financial investment to get started.
• Using lean startup principles or preselling a product to gauge interest is also a good way to reduce initial upfront costs. Noah Kagan of Appsumo is famous for this. This post illustrates his process of creating a profitable idea quickly.

This rather depends on what you consider "investment", "capital investment" and "start".

In theory, a consulting business requires no monetary investment at the point you start the business, but the investment was likely to have been years earlier; a life of education and work experience.

Being a broker could be relatively inexpensive to start - if you can find someone who wants something and someone who has that thing, you can broker the deal, taking a cut, and never taking the financial risk.